[Patch] Fortran: -fno-automatic and -fopenacc / recusion check cleanup

[Tobias Burnus]

Two fixes – simple, see patch + commit text.

Longer description:

* options:
Background:
- OpenMP, OpenACC and imply that a function is called
concurrently and -frecursive implies recusive. In all those
cases, the code may fail if a local variable is in static memory
instead of stack or heap. – If a user specified 'save', we
can assume/hope that they will deal with it - but with
-fno-automatic (→ 'save' implied), the flags clash.

- Additionally, to avoid placing local arrays in static memory,
for -fopenmp/-fopenacc -frecursive and 'unlimited' stack size
use for const-size arrays is implied.

This patch:
- Handle OpenACC as OpenMP (before it didn't imply -frecursive.


* Recursive run-time check. The current code currently generates:

   subroutine foo()
     logical, save :: currently_called = .false.
     if (currently_called) error_stop "No recursive but called"
     currently_called = .true.
     ...
     ... ! Rest of code, which could indirectly call this proc again
     ...
     currently_called = .false.
   end

This works well for recursive calls but less so for concurrency
(→ OpenMP, OpenACC).
As noted above, by default OpenMP/OpenACC implies -frecursive
and, hence, there is no recursive check generated.
The question is what code should be generated for, e.g.
   -fno-automatic -fopenmp or -fopenacc -fmax-stack-var-size=20

In that case, -frecursive is unset. We have two choices:
- Either still always reset, which may not detect concurrent
   use (including recursive + concurrent use) do to a race condition.
- Or avoid resetting the flag
   But then calling the procedure twice (e.g. beginning + end of the
   program) will generate a bogus error.

The current code does the second – at least for -fopenmp.

→ PATCH: Simply use the same condition twice instead of a complicated
   test; do so via: 'if (recurcheckvar == NULL_TREE)'.

Current code:

tree recurcheckvar = NULL_TREE;
...
   if ((gfc_option.rtcheck & GFC_RTCHECK_RECURSION)
       && !is_recursive && !flag_recursive && !sym->attr.artificial)
     ...  // declare 'recurcheckvar', generate error-message code etc.
...
   /* Reset recursion-check variable.  */
   if ((gfc_option.rtcheck & GFC_RTCHECK_RECURSION)
        && !is_recursive && !flag_openmp && recurcheckvar != NULL_TREE)


Comments? Suggestions? – If not, I will commit it as obvious in the next
days.

Tobias

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